Rotors for processing liquid are used, in particular, in centrifugal microfluidics. Appropriate rotors contain chambers for receiving liquid and channels for routing fluid. Under centripetal acceleration of the rotor, the liquid is forced radially outward and may thus arrive at a radially outer position by means of corresponding fluid routing. Centrifugal microfluidics is applied mainly in the field of life sciences, in particular in laboratory analytics. It serves to automate process runs and to perform operations such as pipetting, mixing, measuring, aliquoting and centrifuging in an automated manner.
The centrifugal force used for performing such operations acts radially outward, so that in conventional rotors, liquid is pumped radially outward only, rather than radially inward from a radially outer position to a radially inner position. Thus, the fluidic path and, therefore, also the number of fluidic processes within the rotor are limited by the radius of the rotor. Consequently, studies comprising a large number of fluidic processes may use large rotors which guarantee the radial path that may be used. However, large rotors cannot be employed in standard devices and limit the maximum rotational frequency while, in addition, a large part of the rotor surface area remains unused.
In order to increase the density of fluidic unit operations in such centrifuge rotors, and/or in order to reduce the sizes of centrifuge rotors, it is indispensable to make use of rotors not only in terms of their radial lengths, but also in terms of their surface areas. To be able to realize this, it is advantageous or useful to move sample liquid in centrifuge rotors radially inward, i.e. to pump them inward.
Different techniques of implementing inward pumping within centrifuge rotors are known from conventional technology. Most known techniques utilize active inward pumping, i.e. inward pumping realized by means of external tools.
For example, inward pumping while using an external pressure source is described in Kong et al., “Pneumatically Pumping Fluids Radially Inward On Centrifugal Microfluidic Platforms in Motion”, Letters to Anal. Chem., 82, pp. 8039-8041, 2010.
Thermopneumatic inward pumping of liquid under centrifugation by means of heating air via infrared radiation is described in Abi-Samra et al., “Thermo-pneumatic pumping in centrifugal microfluidic platforms”, Microfluid Nanofluid, DOI 10.1007/s10404-011-0830-5, 2011, and Abi-Samra et al., “Pumping fluids radially inward on centrifugal microfluidic platforms via thermally-actuated mechanisms”, μTAS conference paper, 2011.
In addition, U.S. Pat. No. 7,819,138 B2 describes a microfluidic device wherein liquid is pumped radially inward in idling disc rotors by means of an external air pressure source.
In addition to such active approaches to effecting inward pumping of liquid in centrifugal systems, techniques have been known wherein by using the centrifugal acceleration field acting upon a liquid in a rotating disc, pneumatic energy is produced and stored for later utilization for reversing the flow direction of the liquid when centrifugal acceleration is used. For example, Noroozi et al., “A multiplexed immunoassay system based upon reciprocating centrifugal microfluidics”, Review of Scientific Instruments, 82, 064303 (2011), discloses a fluidics system wherein a pressure chamber is arranged radially inward of a reaction chamber, an air bubble being trapped and compressed within the pressure chamber during centrifugal filling of the reaction chamber at a high rotational frequency. Upon reduction of the rotational frequency, the air bubble within the pressure chamber will expand again, so that a backward movement of the liquid will take place within the reaction chamber. In this manner, efficient mixing is made possible.
In addition, in Noroozi et al., “Reciprocating flow-based centrifugal microfluidics mixer”, Review of Scientific Instruments, 80, 075102, 2009, a method of mixing liquids is known, wherein two inlets of a mixing chamber are fluidically connected to liquid chambers, whereas outlets of the chamber are connected to an air chamber. Upon centrifugal filling of the mixing chamber, air is trapped and compressed within the air chamber. Upon reduction of the rotational frequency, the air trapped within the air chamber expands, so that a backward flow may be produced within the mixing chamber. □y alternately increasing and reducing the rotational frequency, efficient mixing of the liquids within the mixing chamber is to be achieved.
In Gorkin et al., “Pneumatic pumping in centrifugal microfluidic platforms”, Microfluid Nanofluid (2010) 9:541-549, pneumatic pumping in centrifugal microfluidic platforms is described. An inlet chamber is connected to a pressure chamber via a fluid channel which extends radially outward. Under the action of a centrifugal force, which is effected by rotation at a high rotational frequency, liquid is driven from the inlet chamber into the pressure chamber, where an air bubble is trapped and compressed. Upon reduction of the rotational frequency, the air bubble expands again, and the liquid is moved back into the inlet channel. Thus, pumping back of liquid takes place on the same path. In addition, said document describes a further application wherein an outlet chamber is connected to the pressure chamber via a syphon. Given a sufficiently high rotational frequency, the levels of the liquid in the inlet channel, the pressure chamber and the outlet syphon are nearly in equilibrium, while the air volume remaining within the pressure chamber is compressed. Upon reduction of the rotational frequency, the centrifugal force acting upon the liquid becomes smaller, and the compressed air expands, so that liquid is pumped into the inlet channel and into the syphon. In this manner, the syphon may be filled, and the pressure chamber may be emptied into the outlet chamber via the syphon.
In the known methods of inward pumping, tools such as external compressional waves, heating devices or wax valves are thus used, on the one hand. Said tools constitute materials and peripheral devices which are an addition to the rotor, and consequently, they are costly. Moreover, the control of the peripheral devices and the processes within the rotor are complex. Furthermore, these methods are very time-consuming. For example, inward pumping of 68 μl of sample liquid by using an external pressure source takes 60 seconds, as is described by Kong et al., for example. For thermopneumatic pumping as is described, e.g., in Abi-Samra et al., a pumping rate of 7.6±1.5 μl/min is indicated. A further disadvantage of the method in which an external pressure source is used consists in that there is a limited rotational frequency range from 1.5 Hz to 3.0 Hz within which the method works reliably. For thermopneumatic inward pumping, a sealed pressure chamber may be used for the air which is to be heated. Such a pressure chamber has been realized, in the methods described, by melting and solidifying of wax valves, which constitutes an irreversible process, however.
For the method described in U.S. Pat. No. 7,819,138 B2, the rotor is stopped, which may cause undesired inertia and surface effects due to the resulting disruption of the centrifugal force.
Finally, the method described by Gorkin is restricted to returning the sample liquid from the outside to the inside on the same fluidic path back to the original radial position, or to filling a syphon. General inward pumping through a further fluidic path to a position which is radially further inward is therefore not possible.